1. Technical Field
The present disclosure relates to a method for manufacturing a gas sensor integrated on a semiconductor substrate.
The disclosure also relates to a dielectric membrane totally suspended on the semiconductor substrate whereon the gas sensor is integrated.
The disclosure particularly, but not exclusively, relates to a method for manufacturing a gas sensor integrated on a semiconductor substrate, of the type comprising a dielectric membrane suspended on the semiconductor substrate, a heating element and an element sensitive in metallic oxide (also nanostructured), and to the relative suspended dielectric membrane and the following description is made with reference to this field of application for convenience of illustration only.
2. Description of the Related Art
As it is well known, gas sensors comprise an element sensitive to volatile substances present in the environment, a heating element and some metallic electrodes. In particular, the fact that some materials change some of their chemical/physical/electric characteristics when they are in contact with other substances present in the environment surrounding them is exploited. This is the case of the resistivity variation of the material constituting the sensitive element which varies further according to the absorption of these volatile substances. The resistivity variations are measured by means of resistance measurements (voltage/current characteristics) between the electrodes in contact with the sensitive layer. In particular the variations are greater, and thus more easily detectable, when the sensitive element is brought, by the heating element, to a temperature comprised between 200° C. and 500° C. Higher temperatures are however exploited for the desorption of the substances for re-establishing on the sensor the initial conditions after the measurement.
Among the gas sensors, those whose sensitive element is formed by a nanostructured metallic oxide layer are well known, which are particularly suitable for detecting the presence of organic substances present in the surrounding environment. For example, it is known that the resistivity of a metallic oxide, such as the tin oxide (SnO2), varies up to three orders of magnitude if there are hydrocarbons, i.e., organic compounds containing only carbon and hydrogen.
At present, a gas sensor is integrated on a semiconductor substrate, together with the electrodes and the heating element, by means of the known semiconductor technologies. However, the integration on a single “chip” shows some problems, linked to the fact that, for detecting the variations of the electric characteristics of the sensitive element, this latter is at higher temperatures than the environment temperature, typically comprised between 200° C. and 500° C. However, the semiconductor substrate being a good heat conductor, the whole “chip” is subsequently brought to a high operation temperature, which, at the steady state, are practically identical to that of the sensitive element, which determines a considerable energy consumption by the heating element, as well as the malfunction and in some cases the irreversible damage of some of the components of the sensor itself or of other devices if the sensor is integrated with the control electronics or other circuitry.
To overcome this problem, a dielectric membrane is usually realized, serving as thermal insulator of the semiconductor substrate. A first known technical solution consists in realizing a gas sensor integrated on a semiconductor substrate, comprising a dielectric membrane suspended in the air, serving as thermal insulator and realized through the known “bulk micromachining” technique, which consists in the etching of the silicon in alkaline aqueous solutions, for example KOH (potassium hydroxide), NaOH (sodium hydroxide), TMAH (tetramethylammonium hydroxide). The dielectric membrane is then overhung by a heating element, above which the sensitive element is positioned, spaced from the heating element through a protective and insulating layer.
Although advantageous under several aspects, this first solution shows several drawbacks. In fact, the dielectric membrane is very fragile since it is realized through CVD deposition (Chemical Vapor Deposition) of a layer, generally of oxide or nitride or alternated silicon layers of these ones, which shows a high value of mechanical stress.
The fragility of the membrane also prevents from depositing the layer constituting the sensitive element by means of the known “screen printing” or “doctor blade” techniques.
Another drawback is due to the fact that this type of structure is realized through an anisotropic etching carried out from the back of the silicon wafer wherein the sensor is integrated, needing the use of a “double side” lithographic technique and generating, in the silicon itself, a section profile at 54.7°. Consequently, the space of the single device increases a lot. In fact, for realizing, for example, a squared suspended membrane of 100 μm of side on a substrate being 500 μm thick, it would be necessary to open from the back an etching window having a squared surface of more than 800×800 μm2. Then, the space area of the device on the substrate would result much bigger than the one actually useful.
A second known solution consists of a gas sensor realized, instead, by means of the “surface micromachining” technique, which exploits an anisotropic etching of the basic type on the silicon substrate carried out from the front of the silicon wafer. In practice, a selective masking is carried out useful to leave, only in certain desired areas, a dielectric layer previously deposited on the whole silicon wafer. Alternatively, some sacrificial layers are used and some selective chemical etchings are carried out for the release of the membrane.
Although the described solution solves the problem of the increase of the space area, it does not eliminate, however, the problem linked to the stress and to the fragility of the suspended membranes realized by means of the CVD technique.
It is also known that, in the microelectronic industry, to favor the production and commercialization of systems, such as the gas sensors indeed, for the detection and the monitoring of gaseous agents it is important to considerably reduce the costs of the single items constituting them and to increase their reliability, sensitivity, specificity, stability and mechanical strength.
The self-organizing superficial atomic migration technique of the silicon allows to obtain mechanically strong suspended structures and to minimize the space of the devices.
An example of application of this technique is described in U.S. Pat. No. 7,193,256 of STMicroelectronics Srl, the assignee of the present application. In this application a method is described for manufacturing a semiconductor substrate comprising a buried insulating cavity, with the aim of realizing low cost SOI (Silicon On Insulator) structures. This method comprises the steps of: forming a plurality of openings in the semiconductor substrate; forming a superficial layer on the semiconductor substrate so as to superficially close the plurality of openings forming at the same time at least one buried cavity, in correspondence with the openings end distal from the surface. In particular, this cavity is realized starting from opening structures with cylindrical development realized, in the semiconductor substrate, by exploiting the properties of the self-organizing superficial migration process of the silicon.
In practice, after having realized openings with cylindrical development in the substrate, a thermal process is used (“annealing”) at high temperature, for example between 1000° C.-1300° C., in non-oxidizing environment, for example H2, for some tens of minutes.
FIGS. 1A to 1F show the modification steps, further to the thermal process, of the morphology of a opening 11 with cylindrical development realized in a substrate 10, which, as effect of the structural re-organization of the atoms towards minimum energy states, is transformed into a buried spherical cavity 11a, shown in FIG. 1F. FIGS. 2A to 2D show how very close openings 11 are first transformed into cavities of substantially “ninepin-like” shape to become, after, spherical cavities 11a which join the adjacent cavities thus forming a single space or microchannel 11b. In this way, an unlimited number of empty spheres 11a can be connected obtaining multiple geometries, as shown in FIGS. 3 and 4.